Project Summary Adolescence begins a period of vulnerability for depression and suicidality, as well as for sleep loss. Short sleep duration is extremely common in youth (i.e., 44% of high school students report 6 or less hours of sleep on school nights), and is a risk factor for depression, suicidality, and other negative outcomes. Sleep loss is hypothesized to exert an influence on internalizing symptoms by disrupting reward and cognitive control neural pathways. These brain networks continue to develop during adolescence, are implicated in the development and maintenance of depression, and have been shown to be sensitive to acute sleep restriction in our prior experimental work. Our findings of blunted activation within both reward and cognitive control circuitry following sleep restriction (as compared to sleep extension) provide plausible mechanisms by which sleep loss is involved in the pathway to developing internalizing psychopathology. The next critical step to extend this line of work is to examine the cumulative effects of insufficient sleep, to examine sleep-brain relationships naturalistically during the school year, and to examine the extent to which these predict longitudinal changes in depression and suicidality. These are the goals of our proposed study. We propose to enroll a sample of 210 youth ages 11.5? 14.5 (66% female), who are currently in the 7th, 8th, or 9th grades, representing a continuum of habitual sleep duration, but ensuring 50% with 7 or fewer hours of sleep. Participants will then be followed for the next 4 years during the peak risk for depression onset during adolescence, with annual assessments of sleep via actigraphy and behavioral measures (reward learning, cognitive control), along with fMRI scans at baseline (7th?9th grade) and repeated 2 years later (in 9th?11th grade). Self-report internet-based assessments will be collected quarterly (every 3 months; 14?16 in total) to examine more nuanced temporal associations between changes in sleep duration, depression symptoms, and suicidal ideation. In this longitudinal 4-year design, we will examine how declining sleep duration influences reward and cognitive control, and whether sleep and these mechanisms jointly predict subsequent depression symptoms and suicidal ideation. Insufficient sleep may be an important? yet modifiable?vulnerability factor for psychopathology. Understanding the temporal associations linking sleep and mental health?as well as the role of neural mechanisms?will help better target sleep-focused interventions by determining when, and in whom, they would best be directed. Study findings will provide leverage for developing novel interventions, augmenting existing (sleep or mental health) treatments, and/or changing public policy (i.e., later school start times) to improve adolescent sleep, and ultimately, prevent internalizing disorders.